This invention relates to phosphate ester functional fluid compositions and more particularly to compositions containing alkyl phosphate esters and non phosphate aryl compounds useful as aircraft hydraulic fluids.
Functional fluids have been utilized as electronic coolants, diffusion pump fluids, lubricants, damping fluids, bases for greases, power transmission and hydraulic fluids, heat transfer fluids, heat pump fluids, refrigeration equipment fluids, and as a filter media for air-conditioning systems. Hydraulic fluids intended for use in the hydraulic system of aircraft for operating various mechanisms and aircraft control systems must meet stringent functional and use requirements. Among the most important requirements of an aircraft hydraulic fluid is that it be stable against oxidative and hydrolytic degradation at elevated temperatures. Furthermore, such aircraft hydraulic fluids must also maintain low temperature pour point as well as high autoignition temperature, high flash and fire points and acceptable viscosity at a wide range of temperatures
Most aircraft hydraulic fluids used in civilian aircraft contain some combination of phosphate esters including trialkyl phosphates, dialkyl aryl phosphate esters, alkyl diaryl phosphate esters and tri aryl phosphate esters. A hydraulic fluid useful in aircraft is available from applicants' assignee under the trademark Skydrol.®. LD4. This composition typically contains 18 to 25% by weight dibutyl phenyl phosphate, 50 to 60% by weight tributyl phosphate, 4 to 8% of butyl diphenyl phosphate, 5 to 10% of viscosity index improvers, 0.13 to 1% of a diphenyldithioethane copper corrosion inhibitor, 0.005% to about 1% by weight, but preferably 0.0075% to 0.075% of a perfluoroalkylsulfonic acid salt antierosion agent, 4 to 8% by weight of an acid scavenger of the type described in U.S. Pat. No. 3,723,320 and about 1% by weight of 2,6-di-tertiary-butyl-p-cresol as an antioxidant. This composition has proved highly satisfactory in high performance aircraft application.
Commercially successful aircraft hydraulic fluids now in service contain some amount of aryl phosphate esters. However, such fluids must meet new requirements. These requirements include improved fluid life, improved fire resistance and minimum low temperature viscosity. While investigating fluids to meet the new requirements it has been discovered that aryl phosphate esters are not required and, in fact, certain non-phosphate aryl components actually provide improved thermal stability and lower low temperature viscosity.